1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and more particularly to a semi-transmission type liquid crystal display device wherein a single pixel includes a reflection section that displays an image using ambient light, and a transmission section that displays an image using backlight.
2. Description of the Related Art
In a semi-transmission type liquid crystal display device, a single pixel includes a reflection section with a reflective electrode, and a transmission section with a transmissive electrode. In a dark place, the semi-transmission type liquid crystal display device functions as a transmission-type liquid crystal display device that displays an image by selectively transmitting backlight using the transmission section within the pixel. In a light place, the semi-transmission type liquid crystal display device functions as a reflection-type liquid crystal display device that displays an image by selectively reflecting external ambient light using the reflection section within the pixel. Thereby, power consumption can greatly be reduced.
In the semi-transmission type liquid crystal display device, polarization control elements for controlling the polarization state of light are provided, respectively, on outer surfaces of an array substrate and a counter-substrate that constitute a liquid crystal display panel. Each of the polarization control elements is a circular polarization plate that is formed by combining a polarizer plate and two kinds of phase plates (i.e. a ½ wavelength plate that provides a phase difference of ½ wavelength between an ordinary ray and an extraordinary ray with respect to light of a predetermined wavelength, and a ¼ wavelength plate that provides a phase difference of ¼ wavelength between an ordinary ray and an extraordinary ray with respect to light of a predetermined wavelength). In short, each polarization control element controls the polarization state of light of a predetermined wavelength, which is incident on a liquid crystal layer, so that the light may become circularly polarized light.
A birefringent material, of which the phase plate is formed, has such characteristics that a refractive index no of the ordinary ray and refractive index ne of the extraordinary ray depend on wavelengths of light. Accordingly, a retardation value Δn·d of the phase plate (=ne·d−no·d; d is the thickness of the birefringent material in the direction of travel of light) depends on the wavelength of light that is transmitted. Accordingly, in a case where the polarization control element is composed of a polarizer plate and one kind of phase plate, circularly polarized light is produced only when light with a specified wavelength is incident on the polarization control element and a predetermined retardation (e.g. ¼ wavelength) is imparted to the polarization state of linearly polarized light that has passed through the polarizer plate. Hence, in order to produce circularly polarized light by imparting a predetermined retardation in the entire range of wavelengths that are used for color display, it is necessary to use a combination of at least two kinds of phase plates (½ wavelength plate and ¼ wavelength plate), thereby relaxing the wavelength dependency of the retardation value.
However, in the case of the semi-transmission type liquid crystal display device with this structure, reduction in thickness of the entire device cannot be realized due to the thickness of the polarization control element, and the manufacturing cost of the entire device is disadvantageously increased due to the polarization control element that requires a plurality of phase plates.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 01-270024, for instance, proposes a transmission-type liquid crystal display device with a normally black display mode, wherein a polarization control element is formed by combining a polarizer plate and one kind of phase plate (¼ wavelength plate). According to this patent document, the polarization control element is used as a circular polarization plate, and the wavelength dependency is high as mentioned above. It is thus difficult to realize a polarization control element that can impart a predetermined retardation in the entire range of wavelengths that are used for color display, and it is not possible to obtain desired circularly polarized light. Consequently, even if the polarization control element as disclosed in the above-mentioned patent document is applied to the semi-transmission-type liquid crystal display device with a normally white display mode, optical characteristics such as reflectance, transmittance and contrast would considerably deteriorate.